The present invention relates to a method for pivoting plate elements travelling in a machine which processes them and to a device for applying said method, particularly intended for the packaging industry for manufacturing cardboard boxes, for example.
Boxes of this type are made from flat articles or blanks which have been diecut in a previous operation, then folded and glued in a machine commonly called folder-gluer with the aim of obtaining folded boxes which only have to be made up for making good use. Such articles, even partially folded, are called plate elements in the following description according to a generic term.
In order to perform the various operations required for the folding and the gluing of the plate elements, these elements are conveyed within the folder-gluer through various stations by means of roller or belt conveyors.
Depending on the complexity of the boxes to be obtained with this type of machines, it is often necessary to modify the initial orientation of the plate elements by repositioning them. For example, by pivoting these articles on themselves by 90° or 180°, some folding operations of the flaps or edges of the conveyed elements are easier to carry out. Sometimes, due to pivoting the series of processed plate elements by 180°, a second passage in the machine for completing the folding operations can be avoided.
To facilitate such folding operations, U.S. Pat. No. 5,282,528 discloses a conveying device for a so-called double axis folder-gluer, in which two conveyor belts are disposed at right angle to one another. A transfer device is also disposed at the angle of these two conveyors in order to ensure the passage of the plate elements from one conveyor to the other. Since it cannot be operated in a straight line, this system has the drawback of substantially increasing the required space, in comparison with a straight line machine. Moreover, if the plate elements are to be pivoted by 180°, two 90° pivoting systems disposed in succession are needed.
U.S. Pat. No. 4,807,739 proposes to change the orientation of articles with flat contact surfaces by using two conveyor paths disposed side by side, each separately driven at different speeds to cause rotation of the articles which are simultaneously in contact with said two paths. For driving two contact surface portions of a same article at different speeds, it is necessary that the specific mass of said article be sufficient to generate, between each conveyor path and the corresponding surface portion, a friction force able to ensure the drive of each surface portion at a different speed. This is the case when the articles are stacks of sheets, for example. In the case of conveyed articles with low specific mass, hazardous slidings occur and it is impossible to control the pivoting of such elements.
To overcome this drawback, EP 881,173 proposes, for plate elements with low specific mass, a roller conveyor comprising means for changing the orientation of the elements around axes which are perpendicular to the plane of the conveyor. The conveyor is made up of two parallel paths of rollers, mounted above a box in which a suction system is arranged for pressing against the rollers the conveyed elements, which straddle the two paths. Owing to combined facts that the paths are moved at different speeds, that the speeds are adjustable and that the suction increases the friction force of the elements which are simultaneously conveyed on each of these paths, it is possible to change and to control the orientation of the plate elements which travel on this conveyor.
Moreover, one feature of this device is the transverse movement of the chase which bears the two paths with respect to its frame aligned along the longitudinal axis of the machine. The purpose of the movement is to allow relative repositioning of the plate elements on the longitudinal axis before they leave the conveyor. In fact, it has been noted that, depending on their shape, the plate elements are subject to a lateral translation during the pivoting on the conveyor. Consequently, this unexpected translation is taken into account by moving the paths of the conveyor in advance, or the subsequent stations of the machine, of the same amount but in opposite direction.
Although operation is satisfactory for a majority of the processed elements, it has been noted that this roller conveyor doesn't solve a problem which appears when the surface of the conveyed plate elements in contact with the conveyor has an appreciable length/width ratio or is extremely asymmetrical with respect to the longitudinal axis of the machine. Such an asymmetry can be noted, e.g. on some beer boxes or other plate elements having on their contact surface with the conveyor a wide opening greatly offset with respect to the dividing line separating the two roller paths, left and right, from the conveyor.
When a plate element with such features arrives on the conveyor, it begins to pivot then rapidly stops its rotation on itself in an unwanted position, not having finished its rotation of 90° or 180°. The plate element then continues to travel on the conveyor, driven at the speed of only one path while sliding on the other path. Since the plate element is not pivoted enough, it inevitably causes a jam at the outlet of the conveyor, in spite of a position corrector, not shown, which is provided before the downstream folding tools of the machine.